Nowadays many MOEMS (Micro Optical Electro-Mechanical Systems) applications use a tilting motion of a deformable element (e.g., an element used for tilting micro mirrors). State-of-the-art electrostatic actuating devices are configured with an angular degree of freedom and are driven by electrostatic forces. Electrostatic comb drives can be divided to an in-plane comb drive (“IPCD”) and vertical comb drive (“VCD”).
The IPCD and VCD actuators are used to generalize the working principle of a double-sided comb-drive actuator and to obtain an angular motion. They are constructed from two sets of combs, a moving part (henceforth referred to as the rotor) and static part (henceforth referred to as the stator) interlaced together. See, for example, the MOEMS illustrated in FIGS. 1A and 1B. The use of the two combs forms a free space capacitor, whereas the motion between the two combs changes the capacitance of the free-space capacitor thus formed. Accordingly, upon applying a voltage difference between the stator and rotor combs, a vertical electrostatic force is induced that creates a tilt motion. However, in addition to the tilt motion, the applied voltage difference might also produce unwanted vertical and lateral motions of the mirror. FIG. 1C illustrates an example of a schematic cross-sectional view of the pure rotational actuation (about an axis perpendicular to the plane of the illustration). In this example, the forces that cause unwanted vertical and lateral piston motions are canceled by the symmetric layout of the comb stators. The electrostatic forces are generated in the gaps between the combs shown in FIG. 1B. Since the capacitance of a capacitor as described above is mainly influenced by its geometric shape, the dimensions of the combs strongly influence the obtained electrostatic force. A larger electrostatic force can be acquired when the aspect ratio increases (wherein the aspect ratio is defined as the ratio between the height of the comb and the distance between adjacent teeth comprised in the comb).
Several methods are known in the art for fabricating a VCD. These prior art methods can generally be segmented to methods for fabricating the VCD from two device layer wafers, which may be considered as a “self aligned” mode and methods for fabricating each of the combs of the VCD separately, using a different wafer followed by bonding the two (rotor and stator) wafers to create a single VCD. An example of such a self aligned mode for fabricating a VCD is described in U.S. Pat. No. 6,713,367, the disclosure of which is hereby incorporated by reference. The method described by this publication includes the steps of etching in a semiconductor wafer a first comb with a coarse set of teeth, then a second semiconductor wafer is bonded to the first set of teeth, and another set of teeth is etched in the second wafer with teeth overlapping the teeth in the first comb. Even though both top and bottom comb teeth of the VCD actuator are defined by a single fabrication mask, the difficulty of this method lies in the fact that it highly depends upon the accuracy of the machine to obtain sufficiently accurate alignment.
The problems associated with a wafers bonding method are described in length by Jin-Woo Cho, et al., “Electrostatic 1D Micro Scanner with Vertical Combs for HD resolution Display”, Proc. of SPIE, vol. 6466 (2007). Basically a relative small divergence (even one that is within the system's tolerance) in aligning both wafers, might result in electric shortage between overlapping comb's teeth. Thus, current VCD fabrication methods provide a VCD having a relatively low aspect ratio.
There is a need in the art to provide an electrostatic comb drive characterized by having small distances between adjacent comb teeth and by having high accuracy alignment between its two active layers.